The present invention relates to an adjusting method and apparatus for a beam optical system in a scanning microscope that scans and irradiates a charged particle beam to a sample surface and detects emitted secondary charged particles to obtain an observational image. More particularly, the present invention relates to a method and apparatus suited for adjusting a focus correction and an astigmatism correction of a focused ion beam microscope.
The existence of scanning ion microscopes (SIM) and electron microscopes for use as high-magnification microscopes is widely known. Although similar in principle to scanning electron microscopes (SEM), scanning ion microscopes are significantly different in that ions, instead of electrons, are irradiated as a beam to a sample surface, and in that secondary charged particles given off from the sample surface are not limited to only electrons, but include ions as well. Because of the difference in the secondary charged particles given off in using SEM""s and SIM""s, the scanning images also differ in resolution depending on the different types of sample materials being scanned. Accordingly, it is a common practice to select an ion beam microscope when the SIM image is clearer than that of a SEM image for a particular type of sample being examined. Both SEM and SIM images may also be obtained to compare both of the scanned images to enable further detailed observation.
The focused ion beam apparatus has been in use in the semiconductor manufacturing field for about ten years. However, there has been some recent developments in the basic performance of the ion beam optical system. The high-brightness and high-resolution powered systems have become observational microscopes, while the high-accuracy systems have become processing apparatuses used to form and shape the samples.
The scanning microscope operates by using a charged particle beam irradiated onto a sample surface. Secondary charged particles are driven out of the sample surface in the irradiated area. By detecting the secondary charged particles, image information for that area can be obtained. Accordingly, if the beam is spread out and irradiated onto the sample surface, the image information obtained would cover a broader area. When the beam is spread out, the beam energy per unit area is decreased, and the secondary charged particles given off in that particular area are also reduced. Consequently, the image obtained is low in resolution and contrast (signal power/brightness). Also, if the beam is in an elliptical form at a certain direction, that is, oval as opposed to being circular, the area of irradiation will also be elliptical, and the scanned image will have a difference in resolution in that direction, resulting in an image that appears to xe2x80x9cflowxe2x80x9d in that direction with loss of resolution. Therefore, in order to obtain an image with even clarity in every direction, it is required to create a beam that is circular when projected onto the sample surface (that is, circular in sectional form) and which is well-focused onto the sample surface.
A focus adjustment for a scanning microscope of this kind using charged particles has been conventionally available. There is also disclosure in a publication, JP-B-4-40825, of an automatic focus adjusting method for an electron microscope, or the like. This publication teaches one how to perform focus adjustment by sequentially varying a current flowing to an objective lens, while taking scanning images in order to search for the highest point in the detection signal level of the secondary charged particles using a peak detector. When a beam is restricted and irradiated onto a sample surface, the number of secondary charged particles emitted would be high. Therefore, by varying the current flowing to an objective lens and detecting the number of secondary charged particles emitted, the peak level may be detected and automatically set.
Consistent focus adjustment can only be made if the strength of the irradiated beam and the correspondence of the secondary charged particles emitted were always the same. But, it is not possible to perform the desired automatic focus adjustment due to the xe2x80x9cagingxe2x80x9d of the sample. That is, when a focused ion beam is irradiated onto a semiconductor device with a passivation film, a phenomenon is exhibited where in an initial stage of irradiation, the amount of secondary charged particles is great and the image is bright. But, if the scanning is repeated several times, the image may become darker, and then become brighter again. The difference in the intensity of the images is due to the variable amount of secondary charged particles in the sample surface at any given time, even though the strength of the beam is constant. The variance in the number of secondary charged particles makes automatic focusing mechanisms difficult to operate. Moreover, correction of distortions in the beam sectional form, i.e., adjusting for astigmatism correction, requires an adjustment operation skill that simply cannot be performed by a novice operator.
The present invention aims to provide an adjusting method for a charged particle beam optical system that enables focus adjustment that is unaffected by the irradiation beam intensity and the variable nature of the number of secondary charged particles being emitted at any point during irradiation of a sample surface. The method of the present invention allows even a beginner to make focus adjustments, including astigmatism corrections, with less operator training required than in the prior methods.
The present invention relates to a method and apparatus for obtaining an observational image of a sample surface by scanning a charged particle beam to detect secondary charged particles given off from the sample surface. Charged particle beam focusing and astigmatism correction are performed by comparing scanning images: one image obtained from an initial adjusting value, and other images obtained from a xc2x1xcex94 of the initial adjusting value, wherein xcex94 is a predetermined selected value. The clearest image of the images is selected, and the adjusting value of the clearest image is then set as the new initial adjusting value. The entire scanning, comparison, and adjusting process is repeated until an optimal satisfactory image is obtained.
In performing focus correction in the present invention, focusing a beam with a lens of an optical system occurs by varying the power provided to the lens in order to focus the beam onto a sample surface. If the charged particle beam is a focused ion beam, then the intensity of the electric field applied is varied by the power setting of an electrostatic lens (by varying the applied voltage to the lens). If the beam is an electron beam, then the lens power setting (of an electromagnetic lens) is varied by the current supplied to the coil to vary the intensity of the magnetic field. By varying the lens power settings, a focal point f may be changed by xc2x1xcex94f, and the scanning images obtained at f, f+xcex94f, and fxe2x88x92xcex94f may be stored in an image memory, and these three images may be displayed for comparison. It is preferable that these three images are all displayed at the same time, so that direct comparison of these three images may be made by the operator.
If the clearest image of the three is not the image obtained at f, then a new focal point f is shifted by +xcex94f or xcex94f, corresponding to the adjustment distance of the clearest image selected. Then, the entire scanning and comparison process may be repeated, and a new xcex94f may be selected as well, preferably a smaller xcex94f value then the previous value selected (such as one-half of the xcex94f previously used) in order to perform a more precise (fine) adjustment. The process is repeated until an optimal image is obtained, and thus an optimal focal distance is also obtained.
The present invention also allows for astigmatism correction adjustment. Astigmatism in a scanned image occurs as a result of a circular-shaped beam spot becoming oval-shaped. For example, the beam spot may become oval-shaped so that it has a wider diameter in the horizontal direction. When the beam spot is oval and has a wider diameter in the horizontal direction, the beam irradiation becomes more spread out, and the beam irradiation in the vertical direction becomes more narrow. Accordingly, the image information has a lower resolution in the horizontal direction (because the beam intensity is spread out more), and the image information has a higher resolution in the vertical direction (because the beam intensity is more narrow or concentrated). Therefore, in order to obtain an even image all around, it is necessary to adjust the beam spot to form a circular shape. This adjustment is performed by a stigmator of the beam optical system.
The astigmatism adjustment is performed by displaying a plurality of images, one with an initial adjustment value, and then several images having a selected difference or adjustment value from the initial adjustment value. The images are preferably displayed all at once and are visually compared by the operator to select the most even and clearest image from the plurality of images displayed. Because of the nature of correcting an astigmatic focused beam sectional shape into a circular shape, a two-dimensional adjustment is required, which is different than that of the focal adjustment, which only requires a single direction adjustment. Therefore, there are preferably nine images displayed all at the same time, one image scanned from the initial adjustment value, and the remaining eight images scanned from the initial adjustment value in a xcex94 amount in the two directions.
FIG. 7 illustrates a stigmator comprised of an electrode arrangement of an eight-pole astigmatism corrector according to an embodiment of the present invention. By applying voltages to four electrodes A1, A2, A3, and A4, astigmatism correction may be performed in a first direction. By applying voltage to four electrodes B1, B2, B3, and B4, astigmatism correction may be performed in a second direction. In this arrangement, a two-dimensional beam adjustment may be performed.
As illustrated in FIG. 2, images are scanned with a first directional adjustment in a predetermined amount of xc2x1xcex94Sx from the initial adjustment value, and the images are also scanned with a second directional adjustment in a predetermined amount of xc2x1xcex94Sy from the initial adjustment value. Therefore, a total of nine images are scanned and displayed, based on each combination of xc2x1xcex94Sx and xc2x1xcex94Sy from the initial adjustment value, including the image scanned from the initial adjustment value.
As shown in FIG. 2, the scanned images having the following adjustments are preferably arranged in the following order in a three-by-three grid-like manner: (1) top row, left image: Sxxe2x88x92xcex94Sx, Sy+xcex94Sy; (2) top row, center image: Sx, Sy+xcex94Sy; (3) top row, right image: Sx+xcex94Sx, Sy+xcex94Sy; (4) center row, left image: Sxxe2x88x92xcex94Sx, Sy; (5) center row, center image:. Sx, Sy; (6) center row, right image: Sx+xcex94Sx, Sy; (7) bottom row, left image: Sxxe2x88x92xcex94Sx, Syxe2x88x92xcex94Sy; (8) bottom row, center image: Sx, Syxe2x88x92xcex94Sy; and (9) bottom row, right image: Sx+xcex94Sx, Syxe2x88x92xcex94Sy.
The most even and clearest image from among the nine images is selected, and a new initial adjustment value (Sx, Sy) is selected based on the most even and clearest image selected (that is not the center image having the original initial adjustment value). The astigmatism adjustment process may be repeated until the center image is the most even and clearest image, and new xcex94Sx and xcex94Sy amounts may be selected (preferably being values smaller than the xcex94Sx and xcex94Sy amounts used in the previous adjustment) to perform a more precise (fine) adjustment.